Leon Panetta, the US defence secretary, has condemned the "utterly
deplorable" behaviour of several American Marines accused of urinating
on the bloody corpses of Taliban fighters.

Mr Panetta promised those involved would be punished "to the fullest extent".

The incident, which was captured on video and posted online on Tuesday night, united the American and Afghan governments in condemnation yesterday, with Mr Panetta promising a full investigation and swift justice for those responsible.

"I find the behaviour depicted in it utterly deplorable," Mr Panetta said. "This conduct is entirely inappropriate for members of the United States military and does not reflect the standards or values our armed forces are sworn to uphold. Those found to have engaged in such conduct will be held accountable to the fullest extent."

US Secretary of State Hillary Clinton also expressed "total dismay" at the video, which appeared on YouTube, and said it was "absolutely inconsistent with the standards of behaviour that the vast majority of Marines hold themselves to."

Investigators were reported to have identified the unit involved in the video as being from the 3rd Battallion, 2nd Marine Regiment, which is based out of Camp Lejeune, North Carolina.

President Hamid Karzai, already under immense domestic pressure to distance himself from the US, said he was "deeply disturbed" by the footage.

"This act by American soldiers is simply inhuman and condemnable in the strongest possible terms. We expressly ask the US government to urgently investigate the video and apply the most severe punishment to anyone found guilty in this crime," he said. Mr Panetta personally telephoned the Afghan leader to promise a full investigation.

Mr Karzai's comments were echoed by the Taliban, which described the incident as "yet another barbaric act by foreign forces". However, a spokesman for the insurgency said that the video was unlikely to derail delicate back channel negotiations between the Taliban and the US State Department. The two sides are said to be trying to re-establish direct peace talks following the collapse of the most recent diplomatic effort in December.

The US Marine Corps did not question the authenticity of the video and said the investigation would be mounted by the Naval Criminal Investigative Service, its internal law enforcement agency. No arrest have yet been made. A separate internal inquiry will be headed by a Marine general and a senior military lawyer.

General James Amos, commander of the US Marine Corps, said: "Rest assured that the institution of the Marine Corps will not rest until the allegations and the events surrounding them have been resolved."

The 3rd Battallion, 2nd Marine Regiment was named in a caption underneath the 39-second video. This unit of around 1,000 troops was deployed to Helmand province in southern Afghanistan between March and September last year, giving an indication of when and where the footage may have been taken. It is nicknamed the "Betio Bastards" for its role in a battle in the Pacific during the Second World War.

If the video's content is verified, the men featured could face prosecution not only under the US military code but also for war crimes, as the Geneva Convention specifically prohibits the desecration of enemy remains.

The footage was posted on Youtube by an online user named 'semperfiLoneVoice', an apparent reference to the Marine Corps motto "semper fidelis", or "always faithful". In the caption, the user said that the men were members of a sniper scout team, adding: "I thought Marines were supposed to do the right thing when no one is watching." One of the troops is seen holding a sniper rifle.

By yesterday, both the video and the user account had been deleted, sparking online speculation that it may have been put up by a Marine. A military source said that the investigation had not yet been approached by any whistle-blower, but that it would welcome evidence from anyone with knowledge of the incident.

The US has about 20,000 Marines deployed in Afghanistan, mostly in Kandahar and Helmand provinces, with about 90,000 US troops in Afghanistan in total.

US forces have been embroiled in other war zone controversies in recent years, including alleged abuse of detainees at Abu Ghraib prison in Iraq.